Liquid crystal display device treated by uv irradiation

ABSTRACT

A liquid crystal display device includes a pair of substrates, a liquid crystal between substrates and alignment layers disposed on the inner surface sides of the substrates. The alignment layer is made from a material including polyamic acid containing a diamine component and polyimide containing a diamine component different from the diamine component of the polyamic acid. The alignment layer is subjected to alignment treatment by irradiation of light. UV light can be irradiated in the oblique direction onto the alignment layer through a mask having openings. A reflecting plate can be arranged between a UV light source and the mask. Also, bank structures having a thickness from 0.1 to 0.15 μm can be provided on the alignment layer of the TFT substrate.

This is a Continuation of Ser. No. 13/083,715, filed Apr. 11, 2011,which is a Continuation of Ser. No. 12/838,976, filed Jul. 19, 2010, nowU.S. Pat. No. 7,944,532, issued May 17, 2011, which is a Continuation ofSer. No. 12/164,924, filed Jun. 30, 2008, now U.S. Pat. No. 7,782,430issued Aug. 24, 2010, which is a Continuation of Ser. No. 11/820,891filed Jun. 21, 2007, now U.S. Pat. No. 7,430,033, which is aContinuation of Ser. No. 11/450,754 filed Jun. 9, 2006, now U.S. Pat.No. 7,251,001, which is a Divisional of Ser. No. 10/096,182, filed Mar.12, 2002, now U.S. Pat. No. 7,081,935.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid display device in which lightis irradiated onto an alignment layer or bank structures are provided,for controlling the alignment of the liquid crystal. The presentinvention also relates to an exposure apparatus of an alignment layer ofa liquid crystal display device and a treatment method of an alignmentlayer.

2. Description of the Related Art

A liquid crystal display device includes a liquid crystal that issandwiched between a pair of substrates, and an electrode for applyingan electric field to the liquid crystal and an alignment layer forcontrolling the alignment of the liquid crystal are arranged on theinner surface side of each substrate. The alignment layer is treated forrealizing the alignment of the liquid crystal molecules in apredetermined direction. Typically, the alignment layer is rubbed by afiber material such as rayon, and the liquid crystal molecules arealigned in the rubbing direction. When the alignment layer is rubbedwith the fiber material, however, the fiber material scatters, and theliquid crystal panel may be contaminated. Therefore, a technology thatcan control the alignment of the liquid crystal, without rubbing thealignment layer, has been required.

For example, Japanese Unexamined Patent Publication (Kokai) No.11-202336 discloses a technology in which the alignment of the liquidcrystal is controlled by obliquely irradiating the ultraviolet lightonto the alignment layer. In the invention described in thispublication, the ultraviolet light is obliquely irradiated onto thealignment layer through a mask having slits while the alignment layer ismoved.

Japanese Unexamined Patent Publication (Kokai) No. 11-352489 discloses atechnology in which the alignment of the liquid crystal is controlled bydisposing bank structures (projection patterns) on the alignment layer.This technology uses a vertical alignment layer, and the liquid crystalmolecules are aligned vertically to the alignment layer. At positionswhere the bank structure (projection pattern) exists, the liquid crystalmolecules are aligned perpendicular to the side surface of the bankstructure (projection pattern) and generally obliquely to the substratesurface. The bank structure (projection pattern) has side surfaces oneither side thereof. The alignment direction of the liquid crystalmolecules aligned vertically to one of the side surfaces of the bankstructure (projection pattern) is opposite to the alignment direction ofthe liquid crystal molecules aligned vertically to the other sidesurface on the opposite side of the bank structure (projection pattern).In this way, alignment division is achieved.

Alignment division means that one pixel is divided into a plurality ofregions having different alignments of the liquid crystal. In the caseof rubbing, for example, the area of one pixel of the alignment layer isdivided into two regions, the first region is rubbed in one directionand the second region is rubbed in the opposite direction. Thus, theliquid crystal molecules located in contact with the first regionpretilt in one direction and the liquid crystal molecules located incontact with the second-region pretilt in the opposite direction. It isknown that viewing angle characteristics can be improved in the liquidcrystal display device having such an alignment division.

Alignment division can be easily accomplished if a liquid crystal havingnegative dielectric anisotropy and an alignment layer having verticalalignment property are used.

When the ultraviolet light is obliquely irradiated onto the alignmentlayer having a vertical alignment property, alkyl side chains of thealignment layer existing in a plane perpendicular to the irradiationdirection of the ultraviolet light absorb the ultraviolet light and arecut off, and alkyl side chains of the alignment layer existing in aplane parallel to the irradiation direction of the ultraviolet light donot absorb the ultraviolet light and remain as such. The liquid crystalmolecules are thus aligned in accordance with the remaining alkyl sidechains. To achieve alignment division, the ultraviolet light isirradiated obliquely onto one of the regions of the alignment layer inone oblique direction and is irradiated onto the other region of thealignment layer in the opposite oblique direction. In this case, a maskis arranged between the UV light source and the alignment layer so thatthe ultraviolet light can be irradiated selectively onto one region andthe other region.

Various materials are used for the alignment layer. For example,Japanese Unexamined Patent Publication (Kokai) No. 64-004720 discloses aTN type liquid crystal display device using an alignment layercomprising a mixture of polyamic acid and polyimide. In this reference,however, the mixture of polyamic acid and polyimide constitutes a TNtype liquid crystal cell.

Further, as the ultraviolet light is irradiated obliquely onto thealignment layer having a vertical alignment property, the liquid crystalis aligned in a direction parallel to the irradiation direction of theultraviolet light used as the liquid crystal display device. On theother hand, an electrode is disposed with the alignment layer on eachsubstrate. One of the substrates is a TFT substrate having a pluralityof pixel electrodes and bus lines. The other substrate is a color filtersubstrate having a common electrode. The alignment treatment of thealignment layer is conducted in such a fashion that the alignmentdirection of the liquid crystal is parallel to the bus lines. In thiscase, a transverse electric field acts between the pixel electrode andthe bus line, and a problem develops that this electric field disturbsthe alignment of the liquid crystal at the boundary portion between thepixel electrode and the bus line.

In the case where the ultraviolet light is obliquely irradiated onto thealignment layer, the UV light source is disposed at a certain angle tothe alignment layer. In the arrangement having such an angle, thedistance (optical path length) between the UV light source and thealignment layer varies depending on the position of the alignment layer.Therefore, the intensity of the ultraviolet light irradiated onto thealignment layer varies and thus the tilt angle realized thereby mayvary. In consequence, stable alignment cannot be achieved and excellentdisplay cannot be provided as domains appear.

To accomplish alignment division in the alignment control technologyusing UV irradiation, an exposure apparatus including a UV light sourceand a mask is used. The mask is arranged between the UV light source andthe alignment layer so that the ultraviolet light can be selectivelyirradiated onto portions of the alignment layer. In the first method ofalignment division, the area of the opening of the mask is decreased sothat the ultraviolet light having scattering property pass through theopening of the mask. However, this method involves the problem thatutilization efficiency of the ultraviolet light is low and theirradiation time of the ultraviolet light must be elongated.

In the second alignment division method, the area of the opening of themask is set to a size suitable for irradiating a half of the pixel, theUV light source is obliquely arranged relative to the mask and theultraviolet light is irradiated onto a half of each pixel, and then theUV light source is oppositely obliquely arranged relative to the maskand the ultraviolet light is irradiated onto the remaining half of eachpixel. In this case, however, positioning between the mask and thealignment layer becomes more difficult as the size of the pixels becomesmaller. Also, in a proximity exposure, a gap is provided between themask and the alignment layer and their positioning is conducted. As thesize of the pixels become smaller, however, the gap between the mask andthe alignment layer must be decreased, but it is not possible to reducethe space below an allowable value.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a liquid displaydevice capable of accomplishing stable alignment of a liquid crystal andtherefore capable of acquiring excellent display.

It is another object of the present invention to provide an exposureapparatus of an alignment layer that can accomplish stable alignment ofa liquid crystal with high utilization efficiency of ultraviolet light.

It is a further object of the present invention to provide a treatmentmethod of an alignment layer by which positioning between a mask and analignment layer can be easily conducted when an alignment treatment ofthe alignment layer is conducted by means of irradiation of ultravioletlight.

It is a still further object of the present invention to provide aliquid crystal display device in which the disturbance of the alignmentof a liquid crystal due to a transverse electric field at a boundaryportion between a pixel electrode and a bus line can be mitigated.

A liquid crystal display device, according to the present invention,comprises a pair of substrates, a liquid crystal arranged between thepair of substrates and an alignment layer disposed on the inner surfaceside of each of the substrates, wherein the alignment layer comprises amixture of polyamic acid including a diamine component and polyimideincluding a diamine component different from the diamine component ofthe polyamic acid, and the alignment layer is treated for alignment ofthe liquid crystal by irradiation with ultraviolet light.

In this arrangement, polyamic acid has such a property that the achievedpretilt angle hardly changes as the irradiation quantity of theultraviolet light changes, and polyimide has such a property that theachieved pretilt angle greatly changes as the irradiation quantity ofthe ultraviolet light changes. When they are mixed, the pretilt anglecan be accomplished stably in accordance with the mixing ratio ofpolyimide and polyamic acid when a certain time is reached from thestart of irradiation of the ultraviolet light, irrespective of theirradiation quantity of the ultraviolet light. However, if the diaminecomponent used in polyamic acid is the same as the diamine componentused in polyimide, polyamic acid and polyimide are likely to beuniformly mixed, the effect of polyimide to change the alignment statebecomes small, and the property becomes analogous to that of polyamicacid. Therefore, the diamine component used in polyamic acid ispreferably different from the diamine component used in polyimide.

An exposure apparatus of an alignment layer, according to the presentinvention, comprises a ultraviolet light source, a reflecting platehaving slits allowing passage of ultraviolet light irradiated from theultraviolet light source and a reflecting surface on the opposite sideof the ultraviolet light source, and a photomask having openingsallowing passage of the ultraviolet light outgoing from the slits of thereflecting plate and a reflecting surface on the side of the reflectingplate.

In this exposure apparatus, the utilization efficiency of theultraviolet light becomes higher and stable alignment of the liquidcrystal can be accomplished.

A method of treating an alignment layer by irradiating ultraviolet lightonto an alignment layer having a plurality of pixel regions definedtherein to treat the alignment layer for realizing alignment, accordingto the present invention, comprises the steps of arranging a photomaskabove the alignment layer; positioning the photomask with respect to thealignment layer so that a center portion of a first opening of thephotomask is aligned with a center portion of a first pixel region ofthe alignment layer; arranging a ultraviolet light source above thephotomask; wherein the ultraviolet light source, the photomask and thealignment layer are arranged such that the ultraviolet light passingthrough the first opening of the photomask impinges upon a pixel regionof the alignment layer displaced from the first pixel region by n pixelregions (where n is an integer equal to or more than 0); wherein theultraviolet light source, the photomask and the alignment layer arearranged to satisfy the following relationships (1) and (2),

$\begin{matrix}{\left( {{g/2} - 20} \right) \leqq a \leqq \left( {{g/2} + 20} \right)} & (1) \\{{\frac{2d}{\left( {{4n} + 1} \right)\tan \; \theta} - 20} \leqq a \leqq {\frac{2d}{\left( {{4n} + 1} \right)\tan \; \theta} + 20}} & (2)\end{matrix}$

where “a” (μm) is a width of the opening of the photomask, “d” (μm) is agap between the photomask and the alignment layer, θ(rad) is an angle ofthe ultraviolet light made incident to the alignment layer and “g” (μm)is a pitch of the pixel regions: and irradiating the ultraviolet lightfrom the ultraviolet light source to treat the alignment layer forrealizing alignment.

In this treating method, positioning between the mask and the alignmentlayer can be conducted, and the alignment treatment of the alignmentlayer can be securely carried out by the irradiation of the ultravioletlight.

A liquid crystal device, according to the present invention, comprisesfirst and second substrates opposing each other; a liquid crystalarranged between the first and second substrates; a plurality of pixelelectrodes, bus lines and an alignment layer disposed on the innersurface side of the first substrate; and a common electrode and analignment layer disposed on the inner surface side of the secondsubstrate; wherein the alignment layer of at least one of the first andsecond substrates is subjected to treatment for alignment of the liquidcrystal in a predetermined direction, and bank structures having athickness in the rage from 0.1 to 0.15 μm is disposed on the alignmentlayer of the second substrate at positions corresponding to the buslines of the first substrate.

In this construction, the bank structures prevent the disturbance ofalignment of a liquid crystal due to a transverse electric field at theboundary portion between the pixel electrode and the bus line.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the followingdescription of the preferred embodiments, with reference to theaccompanying drawings, in which:

FIG. 1 is a view showing a liquid crystal display device according tothe first embodiment of the present invention;

FIG. 2 is a view showing an example where ultraviolet light, isirradiated onto the alignment layer shown in FIG. 1;

FIG. 3 is, a view showing an example where liquid crystal molecules arealigned when the alignment layer shown in FIG. 2 is used;

FIG. 4 is a view showing a modified example of the liquid crystaldisplay device shown in FIG. 1;

FIG. 5 is a view showing an example where ultraviolet light isirradiated onto the alignment layer shown in FIG. 4;

FIG. 6 is a view showing the relationship between the UV irradiationquantity and the pretilt angle when polyamic acid and polyimide areindividually used to form an alignment layer;

FIG. 7 is a view showing the relationship between the UV irradiationquantity and a pre-tilt angle when polyamic acid and polyimide are mixedand used to form the alignment layer;

FIG. 8 is a view showing the relationship between the UV irradiationquantity and the surface energy of the alignment layer, when two kindsof resins are individually applied to the substrate and when they aremixed and applied;

FIG. 9 is a view showing the relationship between the UV irradiationquantity and the voltage relative retention of the alignment layer, whentwo kinds of resins are individually applied to the substrate and whenthey are mixed and applied;

FIG. 10 is a view, showing an exposure apparatus of an alignment layeraccording to the second embodiment of the present invention;

FIG. 11 is a view showing in detail the photomask shown in FIG. 10;

FIG. 12 is a view showing a modified example of the exposure apparatusof the alignment layer shown in FIG. 10;

FIG. 13 is a view showing the first exposure step of an exposing methodof an alignment layer according to the third embodiment of the presentinvention;

FIG. 14 is a view showing the second exposure step after the firstexposure step shown in FIG. 13;

FIG. 15 is a view showing the substrate having the alignment layertreated by the exposing method shown in FIGS. 13 and 14;

FIG. 16 is a view showing a modified example of the treating method ofthe alignment layer shown in FIGS. 13 and 14;

FIG. 17 is a view showing the width of the non-exposured region and thealignment state of the liquid crystal display device with the alignmentdivision established by exposing twice one pixel region;

FIG. 18 is a view showing the width of the overlapping exposure regionand the alignment state of the liquid crystal display with the alignmentdivision established by twice exposing one pixel region;

FIGS. 19A to 19C are views showing examples of the exposure state of onepixel region when the size of the opening of the photomask is changed;

FIGS. 20A to 20C are views showing examples of the exposure state of onepixel region when a gap between the photomask and the alignment layer ischanged;

FIGS. 21A and 21B are views explaining problems when the pixel pitch ischanged;

FIG. 22 is a view showing a liquid crystal display device according tothe fourth embodiment of the present invention;

FIG. 23 is an enlarged view showing a part of the color filter substrateshown in FIG. 22;

FIG. 24 is a plan view showing the color filter substrate shown in FIG.22;

FIG. 25 is a plan view showing the TFT substrate, with the bankstructures of the color filter substrate shown additionally;

FIG. 26 is a view showing an example of a liquid crystal display devicefor explaining that a transverse electric field disturbs the alignmentof the liquid crystal at a boundary portion between the pixel electrodeand the bus line;

FIG. 27 is a view showing a basic example for preventing the disturbanceof the alignment of the liquid crystal;

FIG. 28 is a view showing the relationship between the overlap of thebank structure with the pixel electrode and the ratio of brightness;

FIG. 29 is a view showing the relationship between the overlap of thebank structure with the pixel electrode and the ratio of brightness; and

FIG. 30 is a view showing the relationship between the overlap of thebank structure with the pixel electrode and the ratio of brightness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be explainedwith reference to the drawings. FIG. 1 shows a liquid crystal displaydevice according to the first embodiment of the present invention. Theliquid crystal display device 10 includes a pair of transparentsubstrates 12 and 14, a liquid crystal 16 arranged between the pair ofsubstrates 12 and 14, and alignment layers 18 and 20 respectively formedon the inner surface sides of the substrates 12 and 14. Electrodes 22and 24 are disposed under the alignment layers 18 and 20, respectively.One of the electrodes is disposed with an active matrix including TFTs.The liquid crystal 16 has negative dielectric anisotropy, and thealignment layers 18 and 20 are Vertical alignment layers. Therefore, theliquid crystal 16 is aligned substantially vertically, with a pretiltwith respect to the substrates 12 and 14, as shown in FIG. 1.

In this embodiment, the alignment layers 18 and 20 are made of amaterial containing polyimide including a diamine component. Preferably,the alignment layers 18 and 20 are made of a mixture of polyamic acidincluding a diamine component and polyimide including a diaminecomponent, the diamine component of polyamic acid being different fromthe diamine component of polyimide. The diamine component of polyimidefurther includes a diamine component that does not substantiallycontribute to vertical alignment and a diamine component thatsubstantially contributes to vertical alignment, in which the proportionof the diamine component that substantially contributes to verticalalignment is equal to or more than 30% to the whole diamine component.

FIG. 2 shows an example where ultraviolet light is irradiated onto thealignment layer 18 (20). It is assumed that the diamine component of thealignment layer 18 (20) that substantially contributes to verticalalignment has alkyl side chains 18 a and 18 b shown with exaggeration.The alkyl side chains 18 a and 18 b protrude with respect to the surfaceof the alignment layer 18 (20) in various directions. The alkyl sidechains 18 a exist in planes substantially parallel to the direction ofincidence of the ultraviolet light, and the alkyl side chains 18 b existin planes substantially perpendicular to the direction of incidence ofthe ultraviolet light. When non-polarized or polarized ultraviolet lightis irradiated obliquely onto the alignment layer 18 (20), the alkyl sidechains 18 b existing in planes substantially perpendicular to theirradiation direction of the ultraviolet light absorb the ultravioletlight and are cut off, but the alkyl side chains 18 a existing in planessubstantially parallel to the direction of incidence of the ultravioletlight do not absorb the ultraviolet light and are not cut off.Therefore, the alkyl side chains 18 a remain uncut.

FIG. 3 shows an example where the liquid crystal molecules are alignedwhen the alignment layer 18 (20) shown in FIG. 2 is used. The liquidcrystal display device including the alignment layers 18 and 20 thustreated is assembled as shown in FIG. 1, and the liquid crystal 16 isinserted. The molecules of the liquid crystal 16 are thus pretilted inaccordance with the alkyl side chains 18 a that remain uncut. Inconsequence, the molecules of the liquid crystal 16 are alignedsubstantially vertically to the substrates 12 and 14 owing to verticalalignment property of the alignment layers 18 and 20 but are pretiltedby a predetermined pretilt angle in accordance with the alkyl sidechains 18 a.

FIG. 4 shows a modified example of the liquid crystal display deviceshown in FIG. 1. In FIG. 4, the liquid crystal 16 is alignedsubstantially vertically with pretilt to the substrates 12 and 14.However, the molecules 16 a of the liquid crystal 16 in one region arealigned (pretilted) in a direction different from that of the molecules16 b of the liquid crystal 16 in another region. It is known that when aplurality of regions having different alignments exist in one pixel(when alignment division exists), the viewing angle characteristic ofthe liquid crystal display device can be improved.

FIG. 5 shows an example where the ultraviolet light is irradiated ontothe alignment layer 18 (20) shown in FIG. 4. In order to cause themolecules 16 a of the liquid crystal 16 in one region to be aligned, theultraviolet light UV1 is irradiated in one oblique direction. In orderto cause the molecules 16 b of the liquid crystal 16 in another regionto be aligned, the ultraviolet light UV2 are irradiated in anotheroblique direction opposite to the oblique direction described above. Amask 26 is used when the ultraviolet light UV1 or UV2 is irradiated.

When the ultraviolet light is irradiated obliquely, the optical lengthbetween the UV irradiation source and the alignment layer 18 (20) variesdepending on the position, and the intensity of the ultraviolet lightirradiated varies depending on the position, with the result that thecaused pretilt angle varies and stable alignment cannot be acquired, asdescribed previously. The present invention solves this problem byconstituting the material of the alignment layer 18 (20) as describedabove.

FIG. 6 is a view showing the relationship between the irradiationquantity of the ultraviolet light and the pretilt angle when polyamicacid and polyimide are individually used for the alignment layers,respectively. Curves A, B and C represent the relationship between theirradiation quantity of the ultraviolet light and the pretilt angle,regarding polyamic acids A, B and C having mutually different contentsof a diamine component contributing to vertical alignment, respectively.For the sake of convenience, the same symbol represents the kind of theresin and the curve corresponding to the former. When polyamic acid isindividually used for the alignment layer, the caused pretilt anglehardly changes even when the content of the diamine component varies.

Curves D, E, F, G, H and I represent the relationship between theirradiation quantity of the ultraviolet light and the pretilt angle,regarding polyimide D, E, F, G, H and I having mutually differentcontents of a diamine component contributing to vertical alignment. Forthe sake of convenience, the same symbol represents the kind of theresin and the curve corresponding to the former. The proportions of, thediamine component contributing to vertical alignment of polyimides D, E,F, G, H and I are 20, 40, 50, 60, 90 and 100%, respectively. Whenpolyimide is individually used for the alignment layer, the pretiltangle achieved by UV irradiation greatly changes. However, whenpolyimide is individually used for the alignment layer, the pretiltangle changes very greatly when the irradiation quantity of theultraviolet light changes.

FIG. 7 is a view showing the relationship between the irradiationquantity of the ultraviolet light and the pretilt angle when thepolyamic acid and polyimide are mixed and used for the alignment layers,respectively. For the sake of convenience, the same symbol representsthe kind of the resin and the curve corresponding to the former. CurvesDA, EA, FC, GB, HA and IA represent the alignment layers prepared bymixing polyamic acids A, B, C and polyimides D, E, F, G, H and Idescribed above, respectively. In the curve DA representing the mixturecontaining polyimide having a small content (20%) of the diaminecomponent contributing to vertical alignment, the pretilt angle greatlyvaries as the irradiation quantity of the ultraviolet light varies. Inthe curve IA representing the mixture containing polyimide having alarge content (90 or 100%) of the diamine component contributing tovertical alignment, the change of the pretilt angle is relatively smalleven when the irradiation quantity of the ultraviolet light becomesgreat. In the curves EA, FC, GB and HA representing the mixturecontaining polyimide having a suitable content (equal to or more than30%) of the diamine component contributing to vertical alignment, thechange of the caused pretilt angle is very small as the irradiationquantity of the ultraviolet light varies.

In summary, polyamic acid has such a property that the achieved changein the pretilt angle is very small even when the irradiation quantity ofthe ultraviolet light is large, whereas polyimide has such a propertythat the pretilt angle varies relatively greatly when the irradiationquantity of the ultraviolet light becomes larger. When these resins aremixed, the characteristics of polyimide and the characteristics ofpolyamic acid are averaged, and a pretilt angle corresponding to themixing ratio of polyamic acid and polyimide is kept stable, after acertain time elapses from the start of irradiation of the ultravioletlight, without depending on the UV irradiation quantity. In other words,in the region of the UV irradiation quantity within 2,000 to 3,000 mJ,the pretilt angle hardly varies in the curves EA, FC, GB and HA evenwhen the UV irradiation quantity varies. Therefore, when the quantity ofthe ultraviolet light actually irradiated changes, the pretilt angleremains substantially constant.

However, if the diamine component used for polyamic acid is the same asthe diamine component used for polyimide, polyamic acid and polyimidebecome miscible more uniformly and the effect of changing the alignmentstate of polyimide becomes smaller, and its property becomes closer tothat of polyamic acid. For this reason, the diamine component used forpolyamic acid is preferably different from the diamine component usedfor polyimide.

Also, there are diamines which can realize a vertical alignment propertyand others which do not realize a vertical alignment property. When theamount of diamine realizing a vertical alignment property is too small,the change of the pretilt upon irradiation of the ultraviolet lightbecomes too great, and pretilt control becomes difficult when polyimideincluding such diamine is mixed with polyamic acid. Therefore, theproportion of the diamine component having vertical alignment propertyto the whole diamine components is preferably equal to or more than 30%.More preferably, the proportion of the diamine component having verticalalignment property to the whole diamine components is equal to or morethan 40% and equal to or less than 90%.

FIG. 8 is a view showing the relationship between the irradiationquantity of the ultraviolet light and the surface energy of thealignment layer, when two kinds of resins are individually applied tothe alignment layer and when their mixture is applied, respectively.Curve B relates to polyamic acid similar to polyamic acid B describedabove. Curve G relates to polyimide G similar to polyimide G describedabove, Curve GB relates to the mixture of polyamic acid B and polyimideG. When the resin whose surface energy hardly changes upon irradiationof the ultraviolet light and the resin whose surface energy greatlychanges are mixed together, a region appears in which the surface energyhardly varies at a certain irradiation quantity, in the same way as inthe case of the pretilt angle shown in FIGS. 6 and 7. In this way, astable pretilt can be obtained.

If the resins have mutually different surface energy values,non-uniformity occurs advantageously to a certain extent when the resinsare mixed, for the same reason as in the case where the diaminecomponents are different. Therefore, the mixture preferably comprisesresins having surface energy values that are different from each otherby a value equal to or more than 2 mN/m on the surface of the alignmentlayers 18 and 20 formed on the substrates 12 and 14, respectively.

Mixing of the resins having different surface energy values changequantities to the ultraviolet light corresponds to mixing of the resinshaving different pretilt angles in the same way as when polyamic acidand polyimide, are mixed. Therefore, it is preferred to form thealignment layers 18 and 20 on the substrates 12 and 14 by mixing atleast two kinds of resins having different surface energy values changequantities to the ultraviolet light.

FIG. 9 is a view showing the relationship between the voltage relativeretention and the irradiation quantity of the ultraviolet light of thealignment layer, when two kinds of resins are individually applied tothe substrate and when their mixture is applied. Curve C relates topolyamic acid C similar to the polyamic acid C described above. Curve Frelates to polyimide F similar to the polyimide F described above. CurveFC relates to the mixture of polyimide F and polyamic acid C. Thealignment layer of curve C has such a property that the voltage relativeretention drops due to the ultraviolet light irradiation, whereas thealignment film of curve F has such a property that the voltage relativeretention rises due to the ultraviolet light irradiation. When bothresins are mixed (curve FC), the drop in the voltage relative toretention can be eventually suppressed. Therefore, an excellent voltagerelative retention can be acquired even when the ultraviolet light isirradiated.

It is therefore preferred to form the alignment layers 18 and 20 on thesubstrates 12 and 14 by mixing at least two kinds of resins the voltagerelative retentions of which, with respect to the ultraviolet light, aredifferent from each other.

Concrete examples will be explained below.

Example 1

Alignment layers 18 and 20 are prepared from a mixture of polyamic acidA and polyimide H (having a 90% content of diamine componentcontributing to vertical alignment). The mixing ratio is polyamic acidA:polyimide H=49:1. The alignment layer material containing this mixtureis applied by using a spinner and is baked to form the alignment layers18 and 20 on the substrates 12 and 14. Ultraviolet light is irradiatedonto the alignment layers 18 and 20 in the oblique direction.Incidentally, several samples are prepared by changing the irradiationquantity of the ultraviolet light. A thermosetting sealant is applied toone of the substrates and spacers of 4 μm are scattered on the othersubstrate, and both substrates are then joined. After vacuum packing,thermosetting is conducted to form an empty cell. A liquid crystalhaving negative dielectric anisotropy is charged into this empty cell invacuum environment to form a liquid crystal display panel. A pretiltangle of the liquid crystal display panel thus fabricated is measured.As represented by curve HA in FIG. 7, the pretilt angle attains minimumat the irradiation quantity 3 J of the ultraviolet light, and the changeof the pretilt angle is very small before and after (near) this minimumvalue. In this way, a stable pretilt angle can be obtained.

Example 2

The alignment layers 18 and 20 are prepared from a mixture of polyamicacid B having a surface energy change shown in FIG. 8 and polyimide G(having a 60% content of a diamine component contributing to verticalalignment). The mixing ratio is polyamic acid B:polyimide G=4:1. Analignment layer material containing this mixture is used to produce aliquid crystal display panel in the same way as in Example 1. A pretiltangle of the liquid crystal panel thus produced is measured. Asrepresented by curve GB in FIG. 7, the pretilt angle attains minimum atan irradiation quantity 3 J of the ultraviolet light, and the change ofthe pretilt angle is considerably small before and after this minimumvalue. In this way, a stable pretilt angle can be obtained. Surfaceenergy hardly changes, either, as shown in FIG. 8.

Example 3

Alignment layers 18 and 20 are prepared from a mixture of polyamic acidC having a′ surface energy change shown in FIG. 9 and polyimide F(having a 50% content of a diamine component contributing to verticalalignment). The mixing ratio is polyamic acid C:polyimde F=3:1. Analignment layer material containing this mixture is used to produce aliquid crystal display panel in the same way as in Example 1. A pretiltangle of the liquid crystal panel thus produced is measured. Asrepresented by curve FC in FIG. 7, the pretilt angle attains about 88°at an irradiation quantity of 2 J to 3 J of the ultraviolet light, andthe change of the pretilt angle hardly exists. In this way, a stablepretilt angle can be obtained. As shown in FIG. 8, the change of thevoltage relative retention is small, and an excellent voltage relativeretention can be acquired.

Comparative Example

Alignment layers 18 and 20 are prepared from a mixture of polyamic acidA and polyimide D (having a 20% content of a diamine componentcontributing to vertical alignment). The mixing ratio is polyamic acidA:polyimide D=49:1. An alignment layer material containing this mixtureis used to produce a liquid crystal display panel in the same way as inExample 1. A pretilt angle of the liquid crystal panel thus produced ismeasured. As represented by curve DA in FIG. 7, the pretilt angleattained drastically drops with the increase of the irradiation quantityof the ultraviolet light, and a stable pretilt angle cannot be acquired.

FIG. 10 shows an exposure apparatus of an alignment layer according tothe second embodiment of the present invention. The exposure apparatus30 of the alignment layer is adapted for the exposure treatment onto thealignment layer 18 (20) of the liquid crystal display device 10 shown inFIG. 4, for example. The exposure apparatus 30 includes a UV lightsource 32, a reflecting plate 34 having slits 34 a allowing passage ofthe ultraviolet light, and a photomask 36 having openings 36 a allowingthe ultraviolet light passing through the slits 34 a of the reflectingplate 34 to directly pass therethrough and the ultraviolet lightreflected by the surface of the reflecting plate 34 on the opposite sideof the UV light source 32 after passing through the slits 34 a of thereflecting plate 34 to pass therethrough.

The slits 34 a of the reflecting plate 34 and the openings 36 a of thephotomask 36 extend in a stripe form in the direction perpendicular tothe sheet of FIG. 10. The slits 34 a of the reflecting plate 34 have awidth of about 5 mm, for example. The openings 36 a of the photomask 36have a width of about 20 μm, for example. The openings 36 a are arrangedin the pitch of 220 μm. The gap between the reflecting plate 34 and thephotomask is 1 cm, and the gap between the photomask 36 and thealignment layer 18 (20) is 100 μm.

In this embodiment, the reflecting plate 34 is attached to a transparentscattering plate 38. A mask used in a prior art exposure apparatus ismade of chromium or the like. Chromium has a low reflection factor. Thereflecting plate 34 and the photomask 36 in this invention are made of amaterial having a high reflection factor in the wavelength region of theultraviolet light. For example, the reflecting plate 34 and thephotomask 36 are made of aluminum or aluminum coated with a fluorinecompound. Alternatively, the reflecting plate 34 and the photomask 36are made of a multi-layered dielectric film.

The UV light source 32 mainly emits the ultraviolet light within therange of 220 to 260 nm. The ultraviolet light within this wavelengthrange is suitable for cutting off the alkyl side chains of the alignmentlayer when irradiated, and can conduct an excellent alignment treatmentof an alignment layer. Since the ultraviolet light in this range can bedistinguished from indoor light, it is easy to handle. Aluminum has areflection factor of about 90% to the ultraviolet light within the rangeof 220 to 260 nm, and is therefore suitable as a material having a highreflection factor for the ultraviolet light.

The surface of aluminum is oxidized during use, and the oxide protectsaluminum but invites a drop in the reflection factor to the ultravioletlight. Therefore, the drop of the reflection factor to the ultravioletlight can be prevented and the reflection factor can be increased ifaluminum, the surface of which is coated with a fluorine compound, isused. Examples of such a fluorine compound include magnesium fluorideand calcium fluoride. Examples of the multi-layered dielectric filminclude SiO₂/MgF₂ and LaF₃/MgF₂. The multi-layered dielectric film isproduced by alternately laminating two materials having mutuallydifferent refractive indices in a thickness of a ¼ wavelength to dozensof layers, and its reflection factor is about 95%.

The surface of the reflecting plate 34 on the side of the photomask 36and the surface of the photomask 36 on the side of the reflecting plate34 serve as the reflecting surfaces. The UV light source 32 emitsscattering light. Therefore, as represented by solid lines with arrows,the ultraviolet light emitted from the UV light source 32 passes throughthe slits 34 a of the reflecting plate 34 and then through the openings36 a of the photomask 36, and impinges upon the alignment layer 18 (20).Further, as represented by broken lines with arrows, the ultravioletlight emitted from the UV light source 32 passes through the slits 34 aof the reflecting plate 34, is reflected by the photomask 36 and then bythe reflecting plate 34, passes then through the openings 36 a of thephotomask 36, and impinges upon the alignment layer 18 (20).

In this way, the ultraviolet light outgoing from the slits 34 a of thereflecting plate 34 directly irradiates the alignment layer 18 (20). Inaddition, the ultraviolet light outgoing from the slits 34 a of thereflecting plate 34 and reflected by the photomask 36 and by thereflecting plate 34 passes through the openings 36 a of the photomask 36and irradiates the alignment layer 18 (20). Therefore, in comparisonwith the case where the reflecting plate 34 does not exist, the quantityof the ultraviolet light passing through the openings 36 a of thephotomask 36 increases, and it becomes possible to acquire the alignmentlayer 18 (20) having high utilization efficiency of the ultravioletlight, and capable of achieving stable alignment of the liquid crystal.As the scattering plate 38 is disposed, the ultraviolet lighttransmitting through the interior of the scattering plate 38 outgoesfrom the slits 34 a of the reflecting plate 34, and utilizationefficiency of the ultraviolet light can be further improved.

The UV light source 32 emits scattering light. The ultraviolet lightpassing through the openings 36 a of the photomask 36 mainly obliquelyirradiates the alignment layer 18 (20). The ultraviolet light (UV1)traveling obliquely in one direction strikes a part of the region of thealignment layer 18 (20). The ultraviolet light (UV2) traveling obliquelyin the opposite direction strikes another part of the region of thealignment layer 18 (20). The function of the alignment treatment of thealignment layer 18 (20), by obliquely irradiating the alignment layer 18(20) with the ultraviolet light, has already been explained withreference to FIGS. 2 and 3. The function of achieving alignment divisionby the ultraviolet light (UV1) traveling obliquely in one direction andthe ultraviolet light (UV2) traveling obliquely in the other directionhas already been explained with reference to FIGS. 4 and 5. Thisembodiment can accomplish alignment division by a single alignmenttreatment.

FIG. 11 shows in detail the photomask 36. The photomask 36 is fabricatedby forming a two-layered structure of a material layer (aluminum) 36 chaving a high UV reflection factor and a material layer (titanium oxide)36 d absorbing the ultraviolet light on a transparent substrate 36 b.The material layer 36 c having a high UV reflection factor reflects theultraviolet light outgoing from the slits 34 a of the reflecting plate34 and allows the ultraviolet light reflected by the reflecting plate 34to pass through the openings 36 a. The material layer 36 d absorbing theultraviolet light is disposed on the side of the alignment layer 18 (20)to be irradiated, and absorbs the ultraviolet light reflected by thealignment layer 18 (20). In this way, the ultraviolet light reflected bythe alignment layer 18 (20) can be prevented from being incident asstray light into the alignment layer 18 (20).

A thermosetting sealant (a product of Mitsui Chemical Co.) is applied toone of the substrates 12 and 14 having alignment layers (a product ofJSR K. K.) 18 and 20 formed by using the exposure apparatus 30 shown inFIGS. 10 and 11, and spacers of 4 μm diameter (a product of Sekisui FineChemical Co.) are scattered on the other substrate. Both substrates arethen joined to each other. After packed in vacuum, the substrates aremaintained in an oven at 135° C. for 90 minutes to fabricate an emptycell. The vertical alignment layer 16 (a product of Merck Co.) havingnegative dielectric anisotropy is charged into this empty cell tofabricate a liquid crystal display panel.

FIG. 12 shows a modified example of the exposure apparatus of thealignment layer shown in FIG. 10. In the same way as in the exampleshown in FIG. 11, the exposure apparatus 30 of the alignment layerincludes a UV light source 32, a reflecting plate 34 having slits 34 aallowing passage of the ultraviolet light and a photomask 36 havingopenings 36 a allowing passage of the ultraviolet light outgoing fromthe slits 34 a of the reflecting plate 34 and the ultraviolet lightreflected by the surface of the reflecting plate 34 on the opposite sideof the UV light source 32. The reflecting plate 34 is attached to atransparent scattering plate 38.

In FIG. 12, the UV light source 32 emits parallel ultraviolet light.Condensing means 40 comprising convex lenses is interposed between theUV light source 32 and the reflecting plate 34. The condensing means 40condenses the ultraviolet light emitted from the UV light source 32 tothe slits 34 a of the reflecting plate 34, and the ultraviolet light socondensed passes through the slits 34 a of the reflecting plate 34. Thelight transmitting through the scattering plate 38 passes through theslits 34 a of the reflecting plate 34, too. The slits 34 a of thereflecting plate 34 have a width of about 5 mm, for example. Theopenings 36 a of the photomask 36 have a width of about 20 μm, forexample, and are disposed in the pitch of 200 μm. The gap between thecondensing means 40 and the reflecting plate 34 is 2 cm, the gap betweenthe reflecting plate 34 and the photomask is 1 cm, and the gap betweenthe photomask 36 and the alignment layer 18 (20) is 100 μm. A liquidcrystal display panel is fabricated in the same way as in the foregoingexample.

FIG. 13 shows the first exposure step of an exposing method of analignment layer according to the third embodiment of the presentinvention. FIG. 14 shows the second exposure step subsequent to the stepshown in FIG. 13. A substrate 14 is a TFT substrate in the same way asthe substrate 14 of the liquid crystal display device shown in FIG. 1.The substrate 14 has pixel electrodes 24 and an alignment layer 20.

Each pixel electrode 24 is surrounded by gate bus lines 42 and data buslines. FIGS. 13 and 14 are sectional views crossing the gate bus lines42. Each pixel electrode 24 has an elongated shape extending in parallelto the data bus lines, and the gate bus lines 42 and the data bus linesdefine a pixel region of 200 μm×70 μm. The gate bus lines 42 and thedata bus lines have a width of 5 μm and are formed in a spaced apartrelationship from the pixel electrode 24 by 3 μm. The opposing substrate12 has a black matrix, the common electrode 22 and the alignment layer18 (see FIG. 1). The black matrix defines a pixel region of 200 μm×70μm. Two gate bus lines 42 determine the pixel pitch g. Similarly, thepixel pitch g is determined in the opposing substrate 12, too.

The exposure apparatus 44 includes a UV light source 46 emittingparallel ultraviolet light and a photomask 48. The photomask 48 includesshading members 48 a (e.g. metallic chromium) deposited on the surfaceof a transparent plate (e.g. quartz). The shading members 48 a formopenings 48 b. The size of the openings 48 b of the photomask 48 is “a”(μm).

The photomask 48 is disposed above the alignment layer 20, and the gapbetween the photomask 48 (the surface of its shading member 48) and thealignment layer 20 is “d” (μm). The photomask 48 is positioned withrespect to the alignment layer 20 in such a fashion that the center ofone opening 48 b of the photomask 48 is aligned with the center of onepixel region (the center between the two gate bus lines 42) of thealignment layer 20.

The UV light source 46 is obliquely disposed above the photomask 48. TheUV light source 46 is disposed in such a fashion that the angle of theultraviolet light made incident to the photomask 48 and to the alignmentlayer 2 is θ (rad). The Uv light source 46 is first disposed obliquelyabove the photomask 48 at the angle θ to conduct the first exposure, asshown in FIG. 13, and is then disposed obliquely and symmetrically tothe UV light source 46 of the first arrangement at the angle θ toconduct the second exposure, as shown in FIG. 14.

The size “a” (μm) of the opening 48 b is preferably about a-half of thepixel pitch “g” (μm). For example, the size “a” of the opening 48 b is100 μm, and the pixel pitch “g” is 200 μm. With this construction, itbecomes possible to position the photomask 48 with respect to thealignment layer 20 and to conduct the first exposure as shown in FIG.13, and to irradiate the ultraviolet light onto the half of each pixelregion of the alignment layer 20. It is also possible to conduct thesecond exposure while the relationship between the photomask 48 and thealignment layer 20 is kept as such (without conducting positioningagain), and to irradiate the remaining half of each pixel region of thealignment layer 20, as shown in FIG. 14. In other words, the troublesomepositioning work between the photomask 48 and the alignment layer 20needs to be carried out only once, and exposure can be conducted twicefrom different angles.

FIG. 15 shows the substrate 14 having the alignment layer 20 that istreated by the exposing method shown in FIGS. 13 and 14. The molecules16 a of the liquid crystal 16 in one region and the molecules 16 b ofthe liquid crystal 16 of another region are aligned in mutually oppositedirections. In this way, the liquid crystal display device havingalignment division can be easily fabricated.

FIG. 16 shows a modified example of the treating method of the alignmentlayer shown in FIGS. 13 and 14. In FIG. 16, the exposure apparatus 44includes the UV light source 46 emitting parallel ultraviolet light andthe photomask 48. The pixel pitch is “g” (μm) and the size of theopening 48 b is “a” (μm). The photomask 48 is disposed above thealignment layer 20, and the gap between the photomask 48 (the surface ofits shading member 48) and the alignment layer 20 is “d” (μm). The UVlight source 46 is obliquely disposed above the photomask 48. The UVlight source 46 is disposed in such a fashion that the angle of theultraviolet light made incident to the photomask 48 and to the alignmentlayer 2 is θ (rad).

The photomask 48 is disposed above the alignment layer 20, and the gapbetween the photomask 48 and the alignment layer 20 is “d” (μm). Thephotomask 48 is positioned with respect to the alignment layer 20 insuch a fashion that the center of one opening 48 b of the photomask 48is aligned with the center of one pixel region (the center between thetwo gate bus lines 42) of the alignment layer 20. The UV light source 46shown in FIG. 16 is first disposed obliquely above the photomask 48 atthe angle θ to conduct the first exposure and is then disposed obliquelyand symmetrically to the UV light source 46 of the first disposition atthe angle θ and conducts the second exposure. In this way, the treatingmethod of the alignment layer shown in FIG. 16 can limit the troublesomepositioning work between the photomask 48 and the alignment layer 20 toonly once, and can conduct the exposure twice from different angles.

In the basic feature of the third embodiment, the UV light source 46,the photomask 48 and the alignment layer 20 are arranged in such afashion that the ultraviolet light passing through one opening 48 b ofthe photomask 48 impinges upon the pixel region that is displaced fromthe pixel region of the alignment layer 20 aligned with this one opening48 b (existing immediately below this one opening 48 b) by n pixelregions (where n is an integer equal to more than 0).

In FIG. 16, the arrangement is such that the ultraviolet light passingthrough one opening 48 b of the photomask 48 impinges upon the pixelregion displaced from the pixel region of the alignment layer 20 alignedwith this one opening 48 b (existing immediately below this one opening48 b) by one pixel region (adjacent pixel region). Incidentally, theexamples shown in FIGS. 13 and 14 correspond to the case where n is 0.

The UV light source 48, the photomask 48 and the alignment layer 20 areso arranged as to satisfy the following relationships.

$\begin{matrix}{\left( {{g/2} - 20} \right) \leqq a \leqq \left( {{g/2} + 20} \right)} & (1) \\{{\frac{2d}{\left( {{4n} + 1} \right)\tan \; \theta} - 20} \leqq a \leqq {\frac{2d}{\left( {{4n} + 1} \right)\tan \; \theta} + 20}} & (2)\end{matrix}$

After positioning is made in this way, the ultraviolet light isirradiated from the UV light source 46 to conduct the alignmenttreatment of the alignment layer 20.

The gap “d” (μm) between the photomask 48 and the alignment layer 20satisfies the following relationship.

$\begin{matrix}{\frac{\left( {{4n} + 1} \right)\left( {a - 20} \right)\tan \; \theta}{2} \leqq d \leqq \frac{\left( {{4n} + 1} \right)\left( {a + 20} \right)\tan \; \theta}{2}} & (3)\end{matrix}$

The angle θ (rad) of the ultraviolet light made incident to thealignment layer 20 satisfies the following relationship.

$\begin{matrix}{{\arctan \; \frac{2d}{\left( {{4n} + 1} \right)\left( {a + 20} \right)}} \leqq \theta \leqq {\arctan \frac{\; {2d}}{\left( {{4n} + 1} \right)\left( {a - 20} \right)}}} & (4)\end{matrix}$

FIG. 17 shows the width of the non-exposed region and the alignmentstate in the liquid crystal display device with the alignment divisionestablished by exposing twice one pixel region. FIG. 18 shows the widthof the overlapping exposed region and the alignment state in the liquidcrystal display device with the alignment division established byexposing twice one pixel region. The non-exposed region and theoverlapping exposed region will be explained with reference to FIGS. 19and 20. If the regions exposed by the first and second exposure stepsare distributed exactly to both sides of the centerline of one pixelregion, the non-exposed region and the overlapped exposed region do notdevelop. In practice, however, the situation is not so ideal. If thenon-exposed region and the overlapping exposed region are small, they donot affect the alignment state. If the non-exposed region and theoverlapping exposed region are large, they do affect the alignmentstate.

FIG. 17 shows that, if the width of the non-exposed region is notgreater than 20 μm, the excellent alignment state can be maintained.FIG. 18 shows that, if the width of the overlapping exposed region isnot greater than 20 μm, the excellent alignment state can be maintained,The formulas (1) and (2) given above are based on the examinationresults shown in FIGS. 17 and 18.

FIGS. 19A to 19C show the exposure state of one pixel region when thesize “a” (μm) of the opening 48 b of the photomask 48 is changed.Reference numeral 50 denotes one pixel region. Reference numeral 50Adenotes the region exposed by the second exposure step. Referencenumeral 50B denotes the region exposed by the first exposure step. Anarrow represents the pre-tilt direction. FIG. 19A shows the case wherethe size “a” (μm) of the opening 48 b is set so that the regions 50A and50B exposed by the first and second exposure steps are distributedexactly to both sides of the centerline of one pixel region. In thiscase, the non-exposed region and the overlapping exposed region do notexist.

FIG. 19B shows the case where the size “a” (μm) of the opening 48 b isset to a value smaller than that in the case of FIG. 19A. In this case,since both exposed regions 50A and 50B become narrower, the non-exposedregion 50C develops. FIG. 19C shows the case where the size “a” (μm) ofthe opening 48 b is set to a value greater than that in the case of FIG.19A. In this case, as both exposed regions 50A and 50B become broader,the overlapping exposed region 50D develops.

FIGS. 20A to 20C show the exposure state of one pixel region when thegap “d” (μm) between the photomask 48 and the alignment layer 20 ischanged. FIG. 20A represents the case where the gap “d” is set so thatthe exposed regions 50A and 50B exposed by the first and second exposuresteps are distributed exactly on either side of the centerline of onepixel region. In this case, the non-exposed region and the overlappingexposed region do not exist.

FIG. 20B represents the case where the gap “d” is set to a value smallerthan that in the case of FIG. 20A. In this case, since the exposedregions 50A and 50B deviate outward, the non-exposed region 50C developsat the center of the pixel and the overlapping exposed region 50Ddevelops at the end of the pixel. FIG. 20C represents the case where thegap “d” is set to a value greater than that in the case of FIG. 20A. Inthis case, since the exposed regions 50A and 50B deviate towards thecenter, the overlapping exposed region 50D develops at the center of thepixel and the non-exposed region 50C develops at the end of the pixel.

FIGS. 21A and 21B are views explaining the problem when the pixel pitch“g” (μm) is changed. In the proximity exposure, positioning between thephotomask 48 and the alignment layer 20 is conducted while the gap “d”is secured between them. As the pixel becomes smaller, however, the gap“d” between the photomask 48 and the alignment layer 20 must bedecreased, but the gap “d” cannot be decreased below an allowable value.

FIG. 21A shows the case where the pixel pitch “g” is sufficiently greatand the gap “d” between the photomask 48 and the alignment layer 20falls within the allowable range. FIG. 21B shows the case where thepixel pitch “g” becomes smaller and the gap “d” must assume a valuesmaller than the allowable range. In practice, however, the gap “d”cannot take a value smaller than the allowable range. In order toconduct two exposure steps by a single positioning step, therefore, itis necessary to keep the gap “d” within the allowable range and tochange the angle θ of the ultraviolet light made incident to thealignment layer. However, when the angle θ of the ultraviolet light madeincident to the alignment layer 20 is changed, the alignment treatmentcapability by the ultraviolet light drops undesirably.

It is advisable in such a case to employ the construction wherein theultraviolet light passing through one opening 48 b of the photomask 48irradiates the pixel region that is displaced from the pixel region ofthe alignment layer 20 aligned with this one opening 48 b (existingimmediately below this one opening 48 b), by n pixel regions (where n isan integer equal to or more than 0), as explained with reference to FIG.16. According to this construction, it is possible to keep the gap “d”within the allowable range and to set the angle θ of the ultravioletlight made incident to the alignment layer 20 to a suitable value. Thethird embodiment can exploit this advantage.

FIG. 22 shows a liquid crystal display device according to the fourthembodiment of the present invention. The liquid crystal display device10 includes a pair of transparent substrates 12 and 14, a liquid crystal16 arranged between the pair of substrates 12 and 14, and alignmentlayers 18 and 20 respectively provided on the inner surface side of thesubstrates 12 and 14. Electrodes 22 and 24 are disposed under thealignment layers 18 and 20, respectively. One of the substrates 12 is acolor filter substrate, and the electrode 22 is a common electrode. Theother substrate 14 is a TFT substrate, and the electrode 24 includes aplurality of pixel electrodes disposed with an active matrix includingTFTs.

FIG. 24 is a plan view showing the inner surface of the color filtersubstrate 12 shown in FIG. 22. The color filter substrate 12 has a blackmatrix 54. FIG. 25 is a plan view showing the inner surface of the TFTsubstrate 14, and bank structures of the color filter substrate 12 areadditionally illustrated in FIG. 25. Gate bus lines 42 and data buslines 52 surround the pixel electrode 24. The data bus lines 52 can beseen in FIG. 22.

The liquid crystal 16 has negative dielectric anisotropy. The alignmentlayers 18 and 20 are vertical alignment layers. The alignment layers 18and 20 are subjected to the alignment treatment so that the liquidcrystal molecules 16 c are aligned in the a direction perpendicular tothe sheet of FIG. 22. In FIGS. 24 and 25, arrows represent the alignmentdirection (pre-tilt direction) of the liquid crystal. The alignmenttreatment can be conducted by using various means. For example, thealignment treatment can be conducted by rubbing or UV irradiation asexplained.

FIG. 23 is an enlarged view showing a part of the color filter substrate12 shown in FIG. 22. In FIGS. 22 and 23, bank structures 56 are disposedon the alignment layer 18 of the color filter substrate 12 at positionscorresponding to the data bus lines 52 of the TFT substrate 14. Adielectric film 58 is formed in a predetermined shape on the commonelectrode 22. The bank structure 56 is formed as a projection of thealignment layer 18 covering the dielectric film 58. The dielectric film58 is extremely thin. When a resist pattern is used to form thedielectric film 58, for example, the resist pattern is made thin byconducting ozone ashing. Alternatively, the resist is in advance dilutedby a diluent and the solution is spin-coated. The resist pattern can bemade thin when the diluent is expelled. In the present invention, thethickness “t” of the bank structures 56 is within the range of 0.1 to0.15 μm.

As can be seen in FIGS. 22 to 25, the bank structures 56 are arrangedimmediately above the data bus lines 52 and extend in parallel to thedata bus lines 52. The width of the bank structures 56 is greater thanthat of the data bus lines 52. Therefore, the bank structures 56 notonly cover the data bus lines 52 but also overlap with the ends of thepixel electrodes 24. Symbol “O” represents the overlapping quantity ofthe bank structure 56 with the pixel electrode 24, and symbol “P”represents the swelling quantity of the bank structure 56 from the blackmatrix 54.

FIG. 26 shows an example of the liquid crystal display device to explaindisturbance of the alignment of the liquid crystal due to a transverseelectric field at the boundary portion between the pixel electrode andthe bus line. The alignment layers 18 and 20 are omitted from FIG. 26.The liquid crystal molecules 16 c are aligned in the directionperpendicular to the sheet of FIG. 26. However, the liquid crystalmolecules 16 d positioned at the boundary portion between the pixelelectrode 24 and the data bus line 52 receive the effect of thetransverse electric field between the pixel electrode 24 and the databus line 52, and are tilted toward the direction parallel to the sheetof FIG. 26 (in the direction from the edge portion of the pixelelectrode 24 towards the center thereof), as represented by broken lineswith arrows. Consequently, the alignment of the liquid crystal isdisturbed at the boundary portion between the pixel electrode 24 and thedata bus line 52, and disclination may occur, as represented byhatching. For example, the transmission drops at this portion when whiteis displayed, and the brightness also drops.

FIG. 27 shows a basic example for preventing the disturbance of thealignment of the liquid crystal. The bank structures 60 having a certainthickness are disposed immediately above the data bus line 52 s inparallel to the data bus lines 52. The liquid crystal molecules 16 d inthe proximity of the bank structures 60 are aligned perpendicular to thesurface of the bank structures 60. The alignment direction of the liquidcrystal molecules 60 in the proximity of the bank structure 60 is inreverse to the alignment direction of the liquid crystal molecules 16 dthat are positioned at the boundary portion between the pixel electrode24 and the data bus line 52 and that receives the effect of thetransverse electric field, and the alignment tendences in two directionsare offset from one another, whereby the liquid crystal molecules areprevented from tilting in a direction parallel to the sheet of FIG. 27(in a direction from the edge towards the center of the pixel electrode24). Consequently, the occurrence of disclination can be suppressed.

However, the bank structures 60 are disposed on the color filtersubstrate 12, and a positioning error at the time of joining the colorfilter substrate 12 and the TFT substrate 14 must be taken intoconsideration. Also, the overlapping quantity between the bank structure60 and the edge of the pixel electrode 24 poses another problem. Whenthe overlapping quantity is small, the alignment controlling force ofthe bank structure 60 becomes small, and fails to suppress theoccurrence of disclination resulting from the transverse electric field.When the overlapping quantity is great, on the contrary, the alignmentcontrolling force of the bank structure 60 becomes great, anddisclination resulting from the bank structure occurs.

As a result of study of the relationship between the overlappingquantity of the bank structure with the edge of the pixel electrode 24and disclination, it is found that the overlapping quantity of the bankstructure 60 with the edge of the pixel electrode 24 that can mostgreatly reduce disclination changes depending on the thickness “t” ofthe bank structure 60. The greater the thickness “t” of the bankstructure 60, the smaller becomes the overlapping quantity that canreduce disclination, and the smaller the thickness “t” of the bankstructure 60, the greater becomes the overlapping quantity that canreduce disclination.

To reduce the occurrence of disclination resulting from the positioningerror, it is preferred to reduce the thickness of the bank structures asmuch as possible. In the present invention, the thickness “t” of thebank structure 56 is in the range of 0.1 to 0.15 μm. In this case, theoverlapping quantity between the bank structure 56 and the edge of thepixel electrode 24, that can reduce disclination, is preferably in therange from 2 to 8 μm.

In the embodiment, the pixel pitch in the direction of the gate buslines 42 is 80 μm, the width of the data bus lines 52 is 5 μm, the gapbetween the pixel electrode 24 and the data bus line 52 is 3 μm, and thewidth of the pixel electrode 24 is 69 μm. The width of the black matrix54 is 11 μm and the pitch of the black matrix 54 is 80 μm. The thicknessof the bank structures 56 is 0.12 μm and the width of the bankstructures 56 is 21 μm. Therefore, the overlapping quantity “O” of thebank structure 56 and the pixel electrode 24 is 5 μm and the swellingquantity of the bank structure 56 from the black material 54 is 5 μm.

Joining of the color filter substrate 12 and the TFT substrate 14 isconducted in such a fashion that the edge of the black matrix 54 isaligned with the edge of the pixel electrode 24. Joining is conductedalso in such a fashion that the pretilt direction of the alignment layer18 of the color filter substrate 12 is reverse to the pretilt directionof the alignment layer 20 of the TFT substrate 14. When the color filtersubstrate 12 and the TFT substrate 14 are joined to each other, apositioning margin of ±3 μm must be taken into consideration. When theoverlapping quantity “O” of the bank structure 56 with the pixelelectrode 24 is 5 μm, the overlapping quantity “O” falls within therange from 2 to 8 μm even when any positioning error occurs. When theoverlapping quantity “O” is within this range, the occurrence ofdisclination can be suppressed.

FIG. 28 shows the relationship between the overlapping quantity (width)“O” of the bank structure 56 with the pixel electrode 24 and thebrightness ratio when the thickness of the bank structures 56 is 0.15μm. The brightness ratio is represented by the brightness at the endportion of the pixel electrode 24 when the brightness at the centerportion of the pixel electrode 24 is 1. The drop of the brightnessremains within 20% when the overlapping quantity “O” is within the rangefrom 2 to 8 μm. The drop of the brightness is small even when theoverlap “O” is outside the range from 2 to 8 μm.

FIG. 29 shows the relationship between the overlapping quantity (width)“O” of the bank structure 56 with the pixel electrode 24 and thebrightness ratio when the thickness of the bank structures 56 is 0.31μm. The drop of the brightness remains within 20% when the overlappingquantity “O” is within a small range around 3 μm as the center. When theoverlapping quantity “O” is outside this range, the brightness dropsdrastically.

FIG. 30 shows the relationship between the overlapping quantity “O” ofthe bank structure 56 with the pixel electrode 24 and the brightnessratio when the thickness of the bank structure 56 is 1.75 μm. The dropof the brightness remains within 20% when the overlapping quantity “O”is in the range from −1 to +3 μm. When the overlapping quantity “O” isoutside this range, the brightness drops drastically. Incidentally, theoverlapping quantity “O” of −1 μm represents that there is a distancebetween the bank structure 56 and the pixel electrode 24.

As explained above, the present invention can accomplish stablealignment of the liquid crystal and can therefore obtain a liquidcrystal display device capable of providing an excellent display.

1. A method of treating an alignment layer, on which a plurality ofpixel regions are defined, for conducting an alignment treatment of saidalignment layer by irradiating ultraviolet light onto said alignmentlayer, said method comprising the steps of: disposing a photomask abovesaid alignment layer; positioning said photomask so that a centerportion of a first opening of said photomask is aligned with a centerportion of a first pixel region of said alignment layer; disposing anultraviolet light source obliquely above said photomask at a firstposition; conducting a first exposure by irradiating ultraviolet lightfrom said ultraviolet light source at said first position; disposing, ata second position, said ultraviolet light source obliquely andsymmetrically to said ultraviolet light source at said first positionabove said photomask; and conducting a second exposure by irradiatingultraviolet light from said ultraviolet light source at said secondposition; wherein a width of a non-exposed region between two regionsexposed by said first exposure and said second exposure is not greaterthan 20 μm or a width of an overlapping exposed region of two regionsexposed by said first exposure and said second exposure is not greaterthan 20 μm.
 2. A method of fabricating a liquid crystal display device,comprising the steps of: forming an alignment layer on a substrate byapplying a mixture of a first material, which substantially contributesto a vertical alignment property of a liquid crystal, and a secondmaterial, which does not substantially contribute to the verticalalignment property of the liquid crystal, to the substrate; disposing aphotomask above the alignment layer; conducting a first exposure byirradiating a pixel region of the alignment layer with ultravioletlight; and conducting a second exposure by irradiating the pixel regionwith ultraviolet light without changing the positioning of the photomaskrelative to the alignment layer.
 3. The method of claim 2, wherein adirection in which the ultraviolet light strikes in the first exposureand a direction in which the ultraviolet light strikes in the secondexposure are different from each other.
 4. The method of claim 3,wherein the direction of the ultraviolet light in the first exposure andthe direction of the ultraviolet light in the second exposure aresymmetrical with respect to a normal of the substrate.
 5. The method ofclaim 3, wherein the direction of the ultraviolet light in the firstexposure and the direction of the ultraviolet light in the secondexposure are each oblique with respect to a normal of the substrate. 6.The method of claim 2, wherein the photomask includes a plurality ofopenings, and a size of each opening is substantially a half of a pixelpitch.